With the rapid development in agriculture and industry, a large amount of pesticides, fertilizers and industrial contaminants have entered into the soil and groundwater. Because soil and groundwater are the resources essential for human survival, the contaminants that reside therein can enter human bodies through food chain and drinking water sources. Unlike surface water that is convenient for centralized processing, the contaminated soil and groundwater are better to be treated by in-situ remediation.
The mechanisms of soil to retain metals can be divided into two major categories: one is to absorb metals on the surface of soil constituents in their ionic state; the other is to form sediment of metallic compounds. At present, one single pollutant rarely exists in the soil environment. Combined pollution of inorganic and organic contaminants poses a threat to human health and ecological safety, and arouses people's concern. Leaching of contaminated soil to wipe out pollutants by use of fluid leacheate is a remediation process that can be performed in situ or ex situ. The liquid leacheate can be water, chemical solvents or other fluids that can leach pollutant from soil, and a leacheate can even be a gas. The in-situ leaching approach does not need to excavate all contaminated soils and transport them to other places for treatment, which not only reduces transportation costs but also avoids the spread of pollutant produced during excavation and transportation of the contaminated soils. The mechanism for electrokinetic remediation of contaminated soil is as follows: the two ends of the contaminated soil are applied with low-voltage direct current (DC) electric field to form a potential gradient; under the effects of electroosmosis and electromigration in the electric field, the contaminants (such as heavy metals or organic pollutants) are migrated to electrode chambers; and then the electrode solution is further processed to remove the contaminants ultimately.
Research on leaching of contaminated soil is mainly focused on surfactants that are used as leacheates. A surfactant can increase the water solubility of organic matters and enhance the removal rate of contaminants. Biosurfactants feature the merits of low cost, easy degradability and large surface activity etc. As a biological metabolite, a biosurfactant does not cause secondary pollution to soil, and the active agents that remain after soil leaching also reduce the nutrition loss from the soil. During the actual leaching process, some technical conditions, such as concentration of leacheate, speed of leaching, quantity of leacheate and ambient temperature, must be considered in order that leaching can achieve optimal results. As a technology combining components from both conventional leaching and electrokinetic techniques, it is necessary to choose conditions, such as soil pH value, that are suitable for both techniques to be used as the end reaction conditions. In addition, specific requirements for each technique also need to be considered. The gist of this invention is to combine multiple remediation techniques and make overall considerations to choose the reaction conditions that can achieve optimal effects.